The present invention relates to a radiocommunications system in which a fixed unit communicates with a set of remote units, fixed and/or mobile, employing code division multiple access (CDMA) and time division multiple access (TDMA) techniques together, in order to implement radio communications. Consequently, various communications can be established simultaneously over a time slot in a frame. The invention proposes a method and a system for distributing the radio resources of the TD-CDMA system, so that radio channel allocation is carried out according to previously established criteria, with the aim that the greatest number of call set-up requests attain their objective and that the channel distribution is optimised at all times.
Generally, a radiocommunications system is connected through fixed units to a transport network such as a public switched telephone network (PSTN), and to the subscribers through remote units, fixed and/or mobile, distributed throughout the coverage area associated with each fixed unit.
The last mile, or final connection, to the subscribers is made over cable, or by radio access, or by a mix of the two techniques. The system has a tree structure, whereby each fixed unit communicates by radio with the remote units that lie within its coverage area.
In order to implement the communications, the radiocommunications system uses code division multiple access (CDMA) and time division multiple access (TDMA) techniques together.
Thus, a radio channel is determined by a carrier frequency, a time slot of a frame and a spread spectrum pseudo-random sequence (since these sequences are orthogonal or quasi-orthogonal with respect to each other, it is to possible to distinguish the radio channels from each other).
It is well known that there are different factors which act negatively reducing the traffic-bearing capacity of the TD-CDMA system, such as interference experienced by a communication in a time slot due to other communications set up in the same time slot, deficient power control, signal attenuation due to the distance between a remote unit and the fixed unit, among others.
When a remote unit makes a call set-up request, the circumstance can arise of this being unsuccessful, even when there are channels free in the cell associated with the fixed unit, for the reason that none of the free radio channels surpass a minimum quality threshold as a consequence of the interference level produced by other radio channels employed in communications already set up within a given time slot of a TD-CDMA frame, for example.
A further drawback arises when a subscriber makes a call set-up request and requires a radio link with a high bit rate for the signal to be transmitted and the TD-CDMA system is unable of attending said request even when there are transmission capacity, because the transmission capacity is distributed inside the TD-CDMA frame. For example, a user requires all the transmission capacity of a time slot, but in all the time slots there are radio channels occupied with calls in process, though not in the maximum possible number. As a consequence, the possibility of failure during the set-up request phase of a communication is increased and the capacity of the TD-CDMA system is diminished.
It can also occur that at a given moment the operational power required by the calls in process is greater than that necessary as a result of the distribution of calls over the different time slots.
That described above represents a drawback for the rapid deployment of TD-CDMA radiocommunications systems, since neither the market nor the operators will press for their implementation.
Therefore, it is necessary to develop a method which permits the distribution of communications already set up within the time slots of the TD-CDMA frame for the purpose of releasing radio channels, in some time slots of the TD-CDMA frame, in order to satisfy the incoming call set-up requests and to optimise the distribution of calls in process, although the above procedure may bring with it the need for handover processes of communications in process without degrading their quality.
In this manner, the users furthest from the fixed unit or with significant path loss, or who require greater transmission capacity, shall encounter their call set-up possibilities improved. The traffic capacity of the TD-CDMA system shall be managed in a more efficient and optimised manner.
An object of the method of the present invention is to facilitate a method that dynamically distributes the radio channels of a TD-CDMA radiocommunications system so that its traffic capacity is maximised, while optimising the quality of links occupied with calls in process. For the purposes of the present invention, the link quality is to be understood in general terms including both the bit rate and the power transmitted. Consequently, a high quality can signify a suitable bit rate according to the particular requirements in each case, or a minimum transmitted power, or a combination of both factors.
Another object of the method of the present invention is that call setup requests be attended in the event that the TD-CDMA system has the theoretical capacity available, without increasing the number of cells necessary that would bring with it an increment of the operational complexity and of the overall cost of the TD-CDMA system.
The radiocommunications system implements duplex communications, and is divided into a plurality of cells each one comprising at least one fixed unit and a set of remote units, fixed and/or mobile, located within the cell. The fixed unit, by means of a radio interface, is linked by radio with the respective radio interfaces of the remote units.
The fixed unit allocates radio channels to the remote units for the purpose of setting up communications over them. A radio channel is defined by a time slot and a pseudo-random sequence, it being possible to set up various simultaneous calls over the same time slot in a TD-CDMA frame.
The method of the present invention manages the distribution of the radio channels of a radiocommunications system in order to optimise the power transmitted, the transmission capacity, or a combination of both factors. According to the method of the invention, a channel is allocated to an incoming call, or to a foreseen incoming call, or to a call in process, comprising the steps of:
a) setting-up of a list of radio channels occupied with calls in process based on said quality requirements;
b) analysis of the quality requirements of a call to which it is desired to allocate a channel;
c) comparison of said analysis with said list in order to find on said list an occupied radio channel in accordance with a predetermined selection criterion;
d) release of said radio channel in said time slot; and
e) allocation to said call of a radio channel within said time slot.
Thus a list of radio channels occupied with calls in process is established; and an analysis of the quality requirements of a call set-up request for incoming, in process or foreseen incoming call is performed.
The result of the analysis performed is compared with the list of occupied radio channels in order to determine which radio channel on the list is to be released. An occupied radio channel is released which belongs to the time slot having the quality closest to that required in the call to which it is desired to allocate a channel. Once the radio channel has been released, the fixed unit allocates a radio channel to the incoming call request within the time slot incorporating the released radio channel.
In accordance with one aspect of the invention, said selection criterion comprises the selection of a channel which corresponds to a time slot having the quality closest to that required in said call to which it is desired to allocate a channel.
In accordance with another aspect of the invention, said selection criterion comprises the selection of a channel which corresponds to a time slot the corresponding channel of which is released with the lowest number of handovers from among the different sets of possible handovers to be made.
In accordance with another aspect of the invention said selection criterion comprises the selection of a channel which corresponds to a first time slot which is situated adjacent in time to a second time slot, forming thereby a double time slot to be allocated to a call.